Method of data acquisition for three-dimensional imaging

ABSTRACT

Described are embodiments of methods of obtaining three-dimensional (3D) surface data of an object scene such as an intra-oral cavity. For example, measured surfaces may include the enamel surface of teeth, the dentin substructure of teeth, gum tissue and various dental structures. The methods can also be applied in medical applications and other applications in which 3D measurement data are acquired with maneuverable 3D measurement devices. In certain embodiments, the measurement device is positioned and translated to enable 3D data to be acquired for a backbone 3D data set. Subsequent controlled motion of the measurement device enables additional 3D data to be acquired and accurately joined to the backbone 3D data set.

RELATED APPLICATIONS

This application is a continuation application of co-pending U.S.application Ser. No. 13/217,595, filed Aug. 25, 2011 and titled “Methodof Data Acquisition for Three-Dimensional Imaging,” which claimspriority to and the benefit of the earlier filing date of U.S.Provisional Application No. 61/381,731, filed Sep. 10, 2010 and titled“Method of Data Processing and Display for a Three-DimensionalIntra-Oral Scanner,” the entireties of which applications areincorporated by reference herein.

FIELD OF THE INVENTION

The invention relates generally to three-dimensional (3D) imaging of theintra-oral cavity. More particularly, the invention relates to a methodof acquiring 3D image data of an object scene using a plurality of 3Dmeasurement scans and generating a complete 3D image from the scans.

BACKGROUND

In a typical dental or medical 3D camera or scanner imaging system, aseries of two-dimensional (2D) intensity images of one or more objectsurfaces in an object scene is acquired where the illumination for eachimage may vary. In some systems, structured light patterns are projectedonto the surface and detected in each 2D intensity image. For example,the projected light pattern can be generated by projecting a pair ofcoherent optical beams onto the object surface and the resulting fringepattern varied between successive 2D images. Alternatively, theprojected light pattern may be a series of projected parallel linesgenerated using an intensity mask and the projected pattern shifted inposition between successive 2D images. In still other types of 3Dimaging systems, techniques such as confocal imaging are employed.

In a dynamic 3D imaging system, a series of 3D data sets is acquiredwhile the camera or scanner is in motion relative to the object scene.For example, the imaging system can be a wand or other handheld devicethat a user manually positions relative to the object scene. In someapplications, multiple objects surfaces are measured by moving thedevice relative to the objects so that surfaces obscured from view ofthe device in one position are observable by the device in anotherposition. For example, in dental applications the presence of teeth orother dental features in a static view can obscure the view of otherteeth. A processing unit registers the overlapped region of all acquired3D data to obtain a full 3D data set representation of all surfacesobserved during the measurement procedure.

SUMMARY

In one aspect, the invention features a method of obtaining 3D surfacedata of an intra-oral cavity. A 3D measurement device having ameasurement field of view is provided. A first scanning step includesmoving the 3D measurement device within an intra-oral cavity so that themeasurement field of view scans across a first surface of a dental arch.During the first scanning step, 3D data for the first surface areacquired by the 3D measurement device. The 3D data for the first surfacedefine a backbone 3D data set for the dental arch. A second scanningstep includes moving the 3D measurement device within the intra-oralcavity so that the measurement field of view scans across a secondsurface of the dental arch. The second surface at least partiallyoverlaps the first surface. During the second scanning step, 3D data forthe second surface are acquired by the 3D measurement device. The 3Ddata for the second surface are geometrically registered with the 3Ddata for the first surface to produce a registration result. Based onthe registration result, the 3D data for the second surface are joinedwith the 3D data for the first surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of this invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which like numerals indicate likestructural elements and features in the various figures. For clarity,not every element may be labeled in every figure. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a block diagram showing an example of a measurement systemthat can be used to obtain a 3D image of an object scene.

FIG. 2 illustrates a maneuverable wand that is part of a 3D measurementsystem used to obtain 3D measurement data for an intra-oral cavity.

FIG. 3 illustrates how an upper dental arch is measured using a handheld3D measurement device such as the maneuverable wand of FIG. 2.

FIG. 4 a flowchart representation of an embodiment of a method ofobtaining 3D surface data for a dental arch according to the invention.

FIG. 5 shows a measurement field of view at five different positionsalong an upper dental arch during 3D data acquisition for an occlusalscan according to the method of FIG. 4.

FIG. 6A and FIG. 6B are an occlusal view and a buccal view,respectively, of a wand and the corresponding location of themeasurement field of view during a scan segment of a buccal surface.

DETAILED DESCRIPTION

The present teaching will now be described in more detail with referenceto exemplary embodiments thereof as shown in the accompanying drawings.While the present teaching is described in conjunction with variousembodiments and examples, it is not intended that the present teachingbe limited to such embodiments. On the contrary, the present teachingencompasses various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art. Those of ordinary skill inthe art having access to the teaching herein will recognize additionalimplementations, modifications and embodiments, as well as other fieldsof use, which are within the scope of the present disclosure asdescribed herein.

The methods of the present invention may include any of the describedembodiments or combinations of the described embodiments in an operablemanner. In brief overview, embodiments of the methods of the inventionenable an accurate 3D measurement of one or more object surfaces. Invarious embodiments described below, the methods relate to theacquisition of 3D data during a 3D measurement procedure. The methodsare described with respect to measurements of an oral cavity, such as ameasurement made by a clinician in a dental application in whichmeasured surfaces may include the enamel surface of teeth, the dentinsubstructure of teeth, gum tissue and various dental structures (e.g.,posts, inserts and fillings). It will be appreciated that the methodscan also be applied in medical applications and other applications inwhich 3D measurement data are acquired with 3D scanning devices underdirect manipulation by an operator or otherwise maneuvered by a controlsystem.

In the embodiments described below, 3D measurement systems usestructured illumination patterns generated by interferometric fringeprojection or other techniques. Imaging components acquire 2D imagesused to determine positional information of points on the surface ofobjects based on the structured illumination of the objects.

U.S. Pat. No. 5,870,191, incorporated herein by reference, describes atechnique referred to as Accordion Fringe Interferometry (AFI) that canbe used for high precision 3D measurements based on interferometricfringe projection. AFI-based 3D measurement systems typically employ twoclosely-spaced coherent optical sources to project the interferometricfringe pattern onto the surface of the object. Images of the fringepattern are acquired for at least three spatial phases of the fringepattern.

FIG. 1 illustrates an AFI-based 3D measurement system 10 used to obtain3D images of one or more objects 22. Two coherent optical beams 14A and14B generated by a fringe projector 18 are used to illuminate thesurface of the object 22 with a pattern of interference fringes 26. Animage of the fringe pattern at the object 22 is formed by an imagingsystem or lens 30 onto an imager that includes an array ofphotodetectors 34. For example, the detector array 34 can be atwo-dimensional charge coupled device (CCD) imaging array. An outputsignal generated by the detector array 34 is provided to a processor 38.The output signal includes information on the intensity of the lightreceived at each photodetector in the array 34. An optional polarizer 42is oriented to coincide with the main polarization component of thescattered light. A control module 46 controls parameters of the twocoherent optical beams 14 emitted from the fringe projector 18. Thecontrol module 46 includes a phase shift controller 50 to adjust thephase difference of the two beams 14 and a spatial frequency controller54 to adjust the pitch, or separation, of the interference fringes 26 atthe object 22.

The spatial frequency of the fringe pattern is determined by theseparation of two virtual sources of coherent optical radiation in thefringe projector 18, the distance from the virtual sources to the object22, and the wavelength of the radiation. The virtual sources are pointsfrom which optical radiation appears to originate although the actualsources of the optical radiation may be located elsewhere. The processor38 and control module 46 communicate to coordinate the processing ofsignals from the photodetector array 34 with respect to changes in phasedifference and spatial frequency, and the processor 38 determines 3Dinformation for the object surface according to the fringe patternimages.

The processor 38 calculates the distance from the imaging system 30 anddetector array 34 to the object surface for each pixel based on theintensity values for the pixel in the series of 2D images generatedafter successive phase shifts of the fringe patterns. Thus the processorcreates a set of 3D coordinates that can be displayed as a point cloudor a surface map that represents the object surface. The processor 38communicates with a memory module 58 for storage of 3D data generatedduring a measurement procedure. A user interface 62 includes an inputdevice and a display to enable an operator such as a clinician toprovide operator commands and to observe the acquired 3D information ina near real-time manner. For example, the operator can observe a displayof the growth of a graphical representation of the point cloud orsurface map as different regions of the surface of the object 22 aremeasured and additional 3D measurement data are acquired.

FIG. 2 illustrates a handheld 3D measurement device in the form of amaneuverable wand 66 that can be used to obtain 3D measurement data foran intra-oral cavity. The wand 66 includes a body section 70 that iscoupled through a flexible cable 74 to a processor and other systemcomponents (not shown). The wand 66 generates a structured light pattern78 that is projected from near the projection end 82 to illuminate asurface to be measured. For example, the structured light pattern 78 canbe an interferometric fringe pattern based on the principles of an AFImeasurement system as described above for FIG. 1. The wand 66 can beused to obtain 3D data for a portion of a dental arch. The wand 66 ismaneuvered within the intra-oral cavity by a clinician so that 3D dataare obtained for all surfaces that can be illuminated by the structuredlight pattern 78.

FIG. 3 illustrates an intra-oral application in which an upper dentalarch is measured using a handheld 3D measurement device such as the wand66 of FIG. 2. Reference is also made to FIG. 4 which presents aflowchart representation of an embodiment of a method 100 of obtaining3D surface data for a dental arch. The measurement results in a complete3D data set that accurately represents the full arch of a patient, i.e.,from the back molar on one side of the arch to the back molar on theopposite side of the arch. During data processing, stitching errors andmotion-induced errors can degrade the measurement results. For a 3Dmeasurement device that includes a 2D imager with a small measurementfield of view (FOV) (e.g., 13 mm×10 mm) relative to the full arch,hundreds of 3D data sets may be stitched together to obtain the complete3D data set for the arch.

According to the method 100, numerous overlapping 3D data sets arestitched together in a common coordinate reference. The 3D data areobtained in a preferred manner or sequence so that the “final” 3D dataset resulting from all the 3D data more accurately represents the dentalarch. In particular, a backbone 3D data set is first generated andadditional sequences of 3D data are subsequently joined to the backbone3D data set. Individual scan segments are used to acquire subsets of 3Ddata for the final point cloud. Limited motion of the wand during the 3Ddata acquisition for each scan segment results in reduced measurementerror and increased measurement accuracy.

A clinician performs the 3D measurement method 100 by positioning (step105) the wand so that the structured light pattern illuminates a portionof an occlusal surface of the dental arch at a starting location, forexample, at one end of the dental arch. 3D data are acquired for theilluminated portion of the occlusal surface. In this example, dataacquisition starts by acquiring data from within a measurement field ofview 86A at the patient's left back molar 90 of the upper dental arch asshown in FIG. 3. The wand is then moved (step 110) from the patient'sleft back molar 90 along the arch to the right back molar whilemaintaining a substantially occlusal view. FIG. 5 shows the measurementfield of view 86A to 86E for several positions (five positions A to E)along the arch. The full occlusal scan does not require significantrotation of the wand during this portion of the 3D measurementprocedure. Rotation and focus induced errors are therefore reduced incomparison to scans obtained by manipulating the wand for other views ofthe arch. In some embodiments, the motion of the wand is substantiallyin a direction parallel to the stripes or fringes of the structuredlight pattern. The primary motion of the wand during the occlusal scanis restricted substantially to a single plane. Advantageously, theocclusal view includes features having substantially higher spatialfrequency content than other views. That is, the occlusal view morereadily shows rapidly varying structure (e.g., gaps between teeth) thatimprove the accuracy for stitching of 3D data relative to other scanviews (i.e., buccal and lingual). Thus the 3D data corresponding to theocclusal scan defines a backbone that can be used for attachment byother 3D data sets obtained during other directional scan views asnecessary to obtain a complete set of 3D data for the arch.

To continue the measurement procedure, the clinician positions (step115) the wand so that the structured light pattern illuminates a portionof the occlusal surface, for example, near or at one end of the arch,and 3D data are acquired that overlap a portion of the backbone 3D dataset. Preferably, the 3D measurement system provides an affirmativevisual or audible indication to the clinician when the new 3D data forthe real-time position of the structured light pattern “locks on” to thedisplay of a point cloud for the backbone 3D data set. Thenewly-acquired 3D data are then registered or joined (step 120) to thebackbone 3D data and serve as the start of a buccal scan segment for thearch. The wand is then rotated about its primary axis and moved (step125) so that a portion of the buccal surface of the arch is illuminatedwith the structured light pattern and 3D data are acquired. The wand isthen maneuvered (step 130) by the clinician so that the structured lightpattern moves along a segment of the buccal surface. For example, thewand may be moved so that the structured light pattern is scanned intime from the patient's back left molars to just beyond the midpoint ofthe buccal surface. FIG. 6A and FIG. 6B show an occlusal view and abuccal view, respectively, of the position of the projection end 82 ofthe wand and the corresponding location of the measurement field of view(and structured light pattern) part way through this scan segment of thebuccal surface. Optionally, the clinician can rotate (step 135) the wandso that the structured light pattern illuminates the midpoint of theocclusal surface. In this manner, the structured light pattern is usedto acquire 3D data in an occlusal view that overlay data in the backbone3D data set to more accurately “register” to the existing 3D data in thecommon coordinate reference system.

A complementary buccal scan segment can now be performed. The clinicianpositions (step 140) the wand so that the structured light patternilluminates a portion of the occlusal surface at the opposite end of thearch and 3D data are acquired that overlap a portion of the backbone 3Ddata set. The 3D data at this location are joined (step 145) to the 3Dbackbone data set and serve as the start of a complementary buccal scansegment for the arch. The wand is then rotated about its primary axis(step 150) so that a portion of remainder of the buccal surface of thearch is illuminated with the structured light pattern and 3D data areacquired. Subsequently, the wand is maneuvered (step 155) by theclinician so that the structured light pattern moves along the remainderof the buccal surface segment. For example, the wand may be moved sothat the structured light pattern is moved in time from the patient'sback right molars to just beyond the midpoint of the buccal surface.Optionally, the clinician can rotate (step 160) the wand so that thestructured light pattern illuminates the midpoint region of the occlusalsurface and 3D data are acquired in an occlusal view that overlay thedata in the backbone 3D data set. Thus the 3D data for this buccalsegment are accurately registered in the common reference system of thebackbone 3D data set.

To complete the 3D measurement of the arch, the clinician obtains 3Ddata for the lingual surface of the dental arch in a manner similar tothat used for obtaining 3D data for the buccal surface. Morespecifically, steps 125 to steps 160 are performed by replacing allreferences to the buccal surface with references to the lingual surface.In total, five scan segments are performed to obtain a full set of 3Ddata for the final 3D data set for the dental arch.

In effect, the steps of joining 3D data to the backbone 3D data setallows sequences of individual 3D images to be attached by referring toa subset of the chronologically ordered 3D images in the backbone 3Ddata set. This joining technique “primes the stitcher” so that thesubsequent scan is properly registered to the backbone 3D data set andaccurately shares the same global coordinate system.

In an alternative embodiment, the order of scan segments can differ. Forexample, acquisition of the 3D data for the two lingual segments canprecede acquisition of the 3D data for the buccal segments.

In other embodiments, the clinician can use a greater number of buccaland lingual scan segments and the extent of each segment can be smaller.In such embodiments, the measurement system displays various portions ofthe backbone 3D data set to allow joining the backbone 3D data set atother locations.

In the embodiments described above for the method 100, the structuredlight pattern and measurement field of view used for 3D data acquisitionare moved along various surfaces of a dental arch and repositioned bymanipulating the position and rotation of a wand. The method can beadapted for other types of 3D measurement systems. For example, themethod can be performed using a measurement field of view of a wand ormaneuverable 3D measurement device that can be translated, rotated andpositioned in a similar manner to the structured light pattern such that3D data initially generated during the procedure can be used to generatea backbone 3D data set and subsequent 3D data can be joined to thebackbone 3D data set to obtain a high accuracy 3D data representation ofan object scene. Furthermore, the method preferably obtains 3Dmeasurement data first for a directional view that substantiallyrequires only two dimensional translation of the measurement field ofview and in which high spatial frequency content is observable to createthe backbone 3D data set and subsequent directional views are used togenerate additional 3D data that can be attached to the backbone 3D dataset.

While the invention has been shown and described with reference tospecific embodiments, it should be understood by those skilled in theart that various changes in form and detail may be made therein withoutdeparting from the spirit and scope of the invention as recited in theaccompanying claims.

What is claimed is:
 1. A method of obtaining three-dimensional (3D)surface data of an intra-oral cavity, the method comprising: providing a3D measurement device having a measurement field of view; a firstscanning step comprising moving the 3D measurement device within anintra-oral cavity so that the measurement field of view scans across afirst surface of a dental arch; during the first scanning step,acquiring, by the 3D measurement device, 3D data for the first surface,the 3D data for the first surface defining a backbone 3D data set forthe dental arch; a second scanning step comprising moving the 3Dmeasurement device within the intra-oral cavity so that the measurementfield of view scans across a second surface of the dental arch, thesecond surface at least partially overlapping the first surface; duringthe second scanning step, acquiring, by the 3D measurement device, 3Ddata for the second surface; geometrically registering the 3D data forthe second surface with the 3D data for the first surface, to produce afirst registration result; and based on the first registration result,joining the 3D data for the second surface with the 3D data for thefirst surface.
 2. A method according to claim 1, further comprising: athird scanning step comprising moving the 3D measurement device withinthe intra-oral cavity so that the measurement field of view scans acrossa third surface of the dental arch, the third surface at least partiallyoverlapping the first surface; during the third scanning step,acquiring, by the 3D measurement device, 3D data for the third surface;geometrically registering the 3D data for the third surface with the 3Ddata for the first surface, to produce a second registration result; andbased on the second registration result, joining the 3D data for thethird surface with the 3D data for the first surface.
 3. A methodaccording to claim 2, wherein the first surface includes an occlusalsurface of the dental arch, the second surface includes a buccal surfaceof the dental arch, and the third surface includes a lingual surface ofthe dental arch.
 4. A method according to claim 1, wherein the firstsurface includes an occlusal surface of the dental arch.
 5. A methodaccording to claim 4, wherein the second surface includes a buccal orlingual surface of the dental arch.
 6. A method according to claim 1,wherein the step of geometrically registering is performed at leastpartially during the second scanning step.
 7. A method according toclaim 1, wherein the second scanning step is performed after the firstscanning step.